1. Field of the Invention
The present invention relates generally to transmission timing control devices, digital roll-off filters and mobile radio terminals in digital radio communications, and in particular to transmission timing control devices controlling a timing for transmission of a transmit signal with reference to a timing of reception of a receive signal, digital roll-off filters limiting a band for transmission of transmit digital data, and mobile radio terminals including the transmission timing control device or the digital roll-off filter in digital radio communications.
2. Description of the Background Art
Conventionally for example in Code Division Multiple Access (CDMA) system or other similar digital radio communications a transmission timing control system is adopted to control a timing for transmission of a transmit signal with reference to a timing of reception of a receive signal. If such a transmission timing control system is used in a digital radio communication a transmitting side is provided with a transmission timing control device controlling a timing for transmission of a transmit signal.
On the other hand, digital data transmission requires a broad transmission band (a frequency band to be occupied) as compared with data rate. Accordingly in digital radio communications a transmission side in view of effective use of radio waves uses a digital filter to limit a band of digital data to be transmitted.
FIG. 16 is a schematic block diagram showing a general configuration of a mobile radio terminal for digital radio communication that is provided with such a transmission timing control device and digital filter.
The FIG. 16 terminal generally includes an antenna 1, a radio processing portion 2, a baseband processing portion 3, an audio input/output device 4 formed of a microphone and a speaker, an external memory 5, a liquid crystal display (LCD), and a display/input device 6 formed of keys.
In particular, base band processing portion 3 includes a modem 3a, a channel codec 3b, a digital signal processor (DSP) 3c, a central processing unit (CPU) 3d, an internal memory 3e, an external interface 3f, and an internal bus 3g. 
Antenna 1 receives a radio wave signal from a base station (not shown). Radio processing portion 2 converts the radio signal to a baseband signal and outputs it to baseband processing portion 3.
In baseband processing portion 3 the received signal is demodulated by modem 3a and furthermore decoded by channel codec 3b and output to DSP 3c. 
DSP 3c processes the received signal in data and drives the speaker of audio input/output device 4 and converts the received signal to a voice.
On the other hand, audio input/output device 4 receives a voice through the microphone. DSP 3c processes the voice in data and outputs it to channel codec 3b, which in turn codes the received audio signal and outputs it to modem 3a, which in turn modulates the received transmit signal and outputs it to radio processing portion 2, which in turn subjects the transmit signal to a radio processing and transmits it through antenna 1 to a base station (not shown).
Note that modem 3a, channel codec 3b and DSP 3c connect through internal bus 3g to CPU 3d, internal memory 3e and external interface 3f. CPU 3d controls an operation of the entirety of the FIG. 16 terminal in accordance with a program stored in internal memory 3e. Furthermore, external interface 3f functions as an interface with external memory 5 and display/input device 6.
The transmission timing control device and the digital filter are respectively used in a mobile radio terminal at a transmitting modem to control a timing for transmission of a transmit signal and to shape a waveform of transmit data (or limit a transmission band). The transmitting modem is configured, as described hereinafter.
FIG. 17 is a functional block diagram showing a transmitting modem portion 30 extracted from modem 3a of baseband processing portion 3 shown in FIG. 16.
Channel codec 3b of FIG. 16 outputs transmit data which is in turn assembled by a radio frame assembling portion 3c into a radio frame and then provided to a spreading modulation portion 30b and spread-modulated.
More specifically in spreading modulation portion 30b a spreading code generated by a spreading code generator (not shown) is multiplexed (or XORed) with transmit data to spreading-modulate the data.
Spreading modulation portion 30b outputs the transmit data spread-modulated to waveform shaping portion 30a which in turn uses a digital filter (not shown) to shape a waveform to limit a transmission occupied band. Waveform shaping portion 30a outputs the transmit data shaped in waveform to radio processing portion 2 of FIG. 16.
Spreading modulation portion 30b of FIG. 17 uses a conventional transmission timing control device, as described in detail hereinafter.
FIG. 18 is a block diagram schematically showing a configuration of a main portion of spreading modulation portion 30b of FIG. 17. Spreading modulation portion 30b mainly includes a transmit data input terminal 101, a spreading portion 102, a timing control portion 103, a chip clock input terminal 104, a receive frame reference signal input terminal 105, a spreading code generation portion 106, and a transmit data output terminal 107.
Radio frame assembling portion 30c of FIG. 17 outputs transmit data which is in turn input through terminal 101 to spreading portion 102 at one input. Furthermore, spreading code generation portion 106 generates a spreading code which is in turn input to spreading portion 102 at the other input.
On the other hand, terminal 104 receives a chip clock which is applied to spreading code generation portion 106, spreading portion 102, and timing control portion 103.
In a CDMA system or any other similar digital radio communications a system is adopted to multiplex (or XOR) transmit data with a predetermined spreading code and transmit it and to allow a recipient side to use a despreading code to despread receive data.
FIG. 19 is timing plots schematically representing a principle of such a spreading process. With reference to FIGS. 18 and 19, transmit data provided to spreading portion 102 through terminal 101 is formed of symbols each serving as a basic unit of transmit data, as presented in FIG. 19(a). For the sake of convenience, Fs represents a frequency of a symbol clock defining the section of each symbol.
Spreading code generation portion 106 is timed in response to a chip clock applied through terminal 104, to generate a predetermined, spreading code sequence, as shown in FIG. 19(b). Note that a spreading code has a basic unit called a chip. For the sake of convenience, Fc represents a frequency of a chip clock defining the section of each chip.
Spreading portion 102 configured of an XOR circuit receives transmit data (FIG. 19(a)) and a spreading code (FIG. 19(b)) which are in turn mutually multiplexed (or XORed) at a timing as defined by a chip clock applied to spreading portion 102 through terminal 104 and a result thereof is output from spreading portion 102 as transmit data, as shown in FIG. 19(c), and received by timing control portion 103.
In general, symbol clock frequency Fs and chip clock frequency Fc has a relation represented by Fc=Fs×N, wherein n is also referred to as a spreading rate. In the FIG. 19 example, spreading rate N is four, although it is not limited thereto and it is selected within a range as determined by the specification of interest, as appropriate.
The transmit data spread by spreading portion 102 is received by timing control portion 103 formed for example of random access memory (RAM), a flip-flop or the like. As has been described previously, in CDMA digital radio communications a transmit signal is transmitted at a predetermined timing with reference to a timing of reception of a receive signal.
For example, TS25.211, a technical specification of the 3rd Generation Partnership Project (3GPP), a third generation mobile communication system, defines that a transmit signal be transmitted with a delay of a time corresponding to T0 (=1024) chips, as counted from a timing of reception of a receive signal.
For example, Japanese Patent Laying-Open No. 11-163766 (H04B 1/707) discloses a CDMA communication system controlling a timing for transmission of a transmit signal, as corresponding to such a definition.
FIG. 20 is timing plots schematically representing a relationship between such a timing of a receive signal and that of a transmit signal as described above. FIG. 20(a) represents a receive signal, which is sectioned by a frame having a length A for example of 10 milliseconds. Furthermore, FIG. 20(b) represents a transmit signal, which is also sectioned by a frame having a length B also for example of 10 milliseconds.
As has been described above, the transmit signal is transmitted after the receive signal is received when a predetermined period C elapses. (According to specification, period C corresponds for example to 1024 chips).
With reference again to FIG. 18, the transmit signal applied from spreading portion 102 to timing control portion 103 is transmitted through terminal 107 at a timing determined by counting a predetermined number of chip clocks 104 (T0=1024 chips for the above specification) applied through terminal 104 with reference to a receive frame reference signal received from a reception modem (not shown) included in mobile radio terminal modem 3a of FIG. 16 (or delayed by period C shown in FIG. 20).
Thus in conventionally controlling a timing for transmission a timing of reception of a receive signal is referenced and thereafter when a chip clock-based time of delay elapses a transmit signal is transmitted.
In digital radio communications a base station is provided with a predetermined clock frequency and so is a mobile radio terminal. However, for example an error of their respective crystal oscillators can gradually offset the frequencies. If this is not corrected, for example the mobile radio terminal would have a timing for transmission of a transmit signal gradually offset relative to a timing of reception of a receive signal.
Correcting such an offset entails adjusting a delay (T0=1024 chips for the above example) in timing for transmission relative to a timing of reception. If this is corrected by shifting a timing for transmission a minimal unit of timing-control or one clock of a chip clock, transmit data would be missing or overlap by one chip.
A conventional digital filter used in wave shaping portion 30a of FIG. 17 will now be described in detail.
In general, when digital data is limited in band it would have a blunt waveform resulting in inter-code interference.
A digital filter for limiting a band without such an inter-code interference is a well known Nyquist filter having Nyquist characteristics, transmission characteristics ideally free of inter-code interference so as to correctly reconstruct original data.
In reality, however, a frequency is hardly cut vertically and a roll-off filter having so-called roll-off characteristics is accordingly used to implement a Nyquist filter.
When an input signal is an impulse signal having an interval T the roll-off filter exhibits frequency characteristics, as represented by the following equation:
                                                                      ⁢              0                                                                        ⁢                              0                <                f                <                                                                            (                                              1                        -                        α                                            )                                        /                    2                                    ⁢                  T                                                                                                                        1                /                2                            ⁢                              {                                  1                  -                                                            sin                      ⁡                                              (                                                                              π                            /                            2                                                    ⁢                          a                                                )                                                              ⁡                                          [                                                                        (                                                      2                            ⁢                            fT                                                    )                                                -                                                  (                                                      1                            -                            α                                                    )                                                                    ]                                                                      }                                                                                                                              (                                          1                      +                      a                                        )                                    /                  2                                ⁢                T                            <              f              <                                                                    (                                          1                      +                      α                                        )                                    /                  2                                ⁢                T                                                                                                      ⁢              1                                                                        ⁢                              f                >                                                                            (                                              1                        +                        α                                            )                                        /                    2                                    ⁢                  T                                                                                        (        1        )            
Furthermore, a root roll-off filter is also used, in which transmitting and recipient sides each use a digital filter having the frequency characteristics represented by equation (1) and together nulls inter-code interference.
The roll-off filter basically operates, as follows: it holds a number of tap coefficients for respective sampling points of a desired impulse response waveform for an impulse input, and input digital data are held by a shift register with a predetermined number of taps in accordance with a sampling period successively, while the data are multiplied by tap coefficients corresponding to their respective tap outputs and then summed up to provide a filter output.
Hereinafter will be described a configuration of a finite impulse response (FIR) filter as one example of a conventional digital roll-off filter.
FIG. 21 is a block diagram schematically showing a conventional FIR filter. Furthermore FIG. 22 represents an impulse response waveform of an FIR filter implementing Nyquist characteristics.
With reference to FIG. 21, input digital data is input for each sampling period Ts successively to a shift register 81 formed of cascade-connected delay elements D1, D2, D3, . . . , Dn in n stages, wherein n represents a positive integer, and the data is delayed by sampling period Ts and thus held by respective delay elements in order. Respective stages of the shift register have their respective outputs, which together n tap outputs of the shift register.
Corresponding to the n tap outputs, n tap coefficients α0′, α1′, α2′, α3′, . . . , αn are previously held in a memory (not shown), the N tap coefficients α0′, α1′, α2′, α3′, . . . , αn corresponding to the values of n sampling points (not shown), respectively, of the predetermined impulse response waveform of FIG. 22.
Then in a sampling period when input digital data are held in the delay elements of the n stages forming the shift register the n tap outputs and the predetermined n tap coefficients α0′, α1′, α2′, α3′, . . . , αn are multiplied respectively, in a multiplication circuit 82 at corresponding n multipliers, respectively, and the resultant n multiplications are added gather by an addition circuit 83 and supplied as the current output of the digital filter.
Then in a subsequent sampling period the digital data held in the delay elements of the n stages are shifted to subsequent stages and in that state the n tap outputs and the predetermined n tap coefficients are multiplied and the resultant multiplications are added together, as described above.
Holding digital data in a shift register, multiplying tap outputs by tap coefficients, and adding the resultant multiplications together can thus be repeated in accordance with a sampling period to provide a digital filter output having inter-code interference free Nyquist characteristics based on the FIG. 22 impulse response waveform
Note that if an input signal is a signal which is 1 or −1 and does not attain 0, i.e., a non-return-to-zero (NRZ) signal then it does not form a train of impulses of 0 or 1. If such an input signal is processed by a roll-off filter it is processed in such a method as described below:
More specifically, by previously converting an input NRZ signal to a normal impulse train and then providing it to the roll-off filter the aforementioned normal filtering process can be provided.
On the other hand, if an input NRZ signal is provided directly to the roll-off filter, the filter uses a tap coefficient corresponding to a value of a sampling point of an impulse response of the frequency characteristic of expression (1) that is multiplied by πft/sin (πft). The tap coefficient thus modified allows the NRZ signal to be filtered directly.
Such a digital roll-filter as described above that is associated with larger numbers of sampling points on an impulse response wave desired, i.e., larger numbers of tap coefficients can provide more ideal output waveforms. However, the shift register would accordingly have increased number of stages and furthermore the multiplication circuit and the addition circuit would also be increased in size.